Exhaust Manifolds: Gasket or no Gasket? + Harmonic Dampers


New member
I am hoping someone can share a little bit of their experience on a couple of topics I'm trying to learn more about, First is exhaust manifolds on the 300. I see that there are a large number of different manifold gaskets available, some in sets, and some as 1 large gasket for intake and exhaust. Ford originally used no gasket on the exhaust side and so the exhaust flange was a little thicker than the intake maniold flange at the head to compensate. My question is, have any of you experienced problems with leaking or shifting manifolds due to the uneven clamping force when running a 1 piece gasket? If I get my EFI manifolds checked for warpage, should I have the flanges shaved down to match the thickness of the intake, then run a gasket on both? Or alternatively, should I run no exhaust gasket at all? I've been told that the gasket creates an undesireable thermal barrier between the head and the manifolds.

The second thing is a question about the stock harmonic damper. Would you consider an aftermarket damper (of any design) to be a worthwhile addition to a mildly built six? I was thinking about the fact that the damper is "tuned" from the factory to dampen crank harmonics in a particular RPM range where the engineers know they will happen. Is it possible that by altering the weight of the rotating assembly (lighter pistons, lighter rods, using a flywheel thats 10 lbs lighter) I could "move" that bad vibration to an RPM where the stock damper will be ineffective? I'd be curious to know where in the rev range those vibrations are occuring. Or will I be perfectly fine with a new, stock damper as long as I keep it below 5000 RPM? Ok, I've finally finished typing :lol: Feel free to tell me I'm crazy if I'm overthinking this!

Nope your not overthinking or typing

The unequal thicknesses of mounting tabs has always bugged me. I have and do use the single piece gasket, but would love to hear other opinions about this .


Active member
When I get ready to install my offy and HD manifolds, since I'll need them planed once they're bolted together, I'm going to see how much the machine shop will charge to mill the exhaust mounting flanges to match the intake.
I've been told, though uncorroborated that perfectly flat surfaces of like material need no gasket between them

Is there truth to this?


Well-known member
Lots of parts are now sealed with anerobic sealers since the surfaces are machined to close tolerances. Ford used a single gasket on the intake side of these manifolds and a large metal washer that was slightly concave to fasten the manifolds to the head.


Well-known member
My setup is still original '67 240cid , but i was faced with the problem of a warped exhaust manifold. The original as you know is both manifold are bolted together. I Had them machine both together as a whole, they are both even with each other now. The manifold gasket was a Felpro with the intake gasket riveted on top of a multilayer steel gasket that covered both exhaust and intake, making them un-even. I removed the intake layer leaving just the steel, Then applied an even, thin layer of RTV around the runners on the intake manifold and bolted up the assembly. Its been working excellent. I did install super thick washers from a fastener house to help clamping distribution
I'm eager to know what you end up doing and how well it does; I too am wanting to install some efi exhaust manifolds


Staff member
VIP Member
FTF did a great post (here on the Ford six site) with pictures somtime back on this problem when using the EFI Exhaust Manifolds with the Offy or Cliford Aluminun Intakes plus the FelPro intake / exhaust gasket. The thick washers will need to be stepped so that they can contact both of the intake and exhaust manifold ears flat to apply pressure evenly otherwise there is a chance of breaking an ear or ears off the aluminum intakes. Also be sure when torquing the bolts down work from the center to outside in two or three steps to their final torque setting. I think for your build the stock Dampner should work great. Good luck (y) :nod: Edited


Well-known member
I've pulled and installed my Clifford intake and shorty headers probably 2 dozen times in the last 6 years for all different reasons and gasket blowouts. I've ran the 1 layer mr gasket, the felpro 2 layer and the felpro with the paper intake layer removed (only the exhaust layer). They all worked perfect until a few months later my warped headers blow a hole through somewhere. If the exhaust is a bit warped and you use the fel pro gasket beware it can blow the exhaust gasket causing a bunch of small pinhole leaks addong up to one big leak you can't see until you take it apart. My bote is for the mr gasket one piece since it's also a bit soft. Be sure to go back and torque them several times over the first few days in use.

Be sure to use a gasket though. You can add some permeatex copper sealant to the exhaust gasket if you like. For the different thicknesses between intake and exhaust tabs I've remedied this by doing one of two things to some of the bolts. 1. Take the factory thick washer put it in a vice and grind half the washer thinner so it's a stepped washer. 2. Put a fat hardware store washer under the factory thick washer so that the fat washer bends around the tabs.
Someone should dig up that write up on fasteners . If memory is accurate it also discussed " hangers" that supported the manifolds from below. I can't find it

For gaskets :
The only way to fly
http://catalog.remflex.com/FORD_Header_ ... p/3005.htm

Reminding all that the factory efi intake torque setting is very sensitive.
Unsure if this is with a motorcraft gasket or any gasket


Well-known member
Interesting read. I have contemplated how hard it was going to be to get all this stuff lined up and fastened. I found a thread on FTE regarding the part numbers on the studs and hardware from a online hardware house. Frenchtown Flyer I believe was in the thread mentioning the clips.....I’ll try to find a link if I can. Still haven’t decided on exhaust manifold paired with the Offy C, hope I don’t end up having to modify too much.

I was incorrect about the manifold supports. Here’s the thread in question.
https://www.ford-trucks.com/forums/www. ... ?t=1383312
If engine is out of vehicle I like to lay her on her side on an engine stand and assemble everything that way .